It is well known that the phenomenon of superconductivity is destroyed by raising the temperature of a superconducting material above its critical temperature T.sub.c. It is also well known that by exposing a superconductor to a magnetic field or by applying too strong a current density, a superconductor will lose its superconducting properties. The threshold current density J.sub.c and the threshold magnetic field H.sub.c (also known as the critical current density and critical magnetic field) necessary for destroying superconductivity at a temperature below T.sub.c have been found to be a function of temperature. That is, as the temperature of a superconductor is lowered below its critical temperature T.sub.c, the critical current density J.sub.c and critical magnetic field H.sub.c increase in magnitude. Thus, as the temperature is lowered, the superconductor is capable of conducting increased electrical current and may be exposed to a stronger magnetic field without adversely impacting its superconducting properties.
A superconducting material with a relatively high critical temperature T.sub.c will exhibit a high critical current density J.sub.c and high critical field H.sub.c. Also, a material with a higher critical temperature T.sub.c requires less cryogenic support to obtain the same performance in comparison to low T.sub.c materials. Modern research has yielded superconducting materials with reported critical temperatures T.sub.c reaching 160 Kelvin. As scientific research yields superconducting materials with ever increasing critical temperatures, the potential for practical applications for superconductors increases with the ultimate goal being superconductor technology at room temperature.
A ferrite is an iron oxide-based material that combines dielectric properties with an internal magnetization that is created when it is energized by an externally applied magnetic field. Magnetic media such as ferrites are composed of ions which possess microscopic magnetic dipoles. Ordinarily the dipoles are randomly oriented so that the bulk magnetic properties are weak or absent. When a magnetic specimen is immersed in an externally applied magnetic field H, the dipoles tend to align with the magnetic field H, and the interior of the material takes on a resultant magnetic moment density or magnetization M. The vector combination of H and M is the magnetic flux (density) B. The concept of magnetic flux implies two components, one from an external magnetic field and the other from an internal magnetization, with either or both being present at any time.
Depending on the particular shape of the magnetic structure, magnetic dipoles may point perpendicular to the surface of the structure, giving rise to north and south magnetic poles. The poles act as sources of an induced magnetic field generally distributed both inside and outside of the structure. Since the internal induced field is directed opposite to the magnetization, the magnetization will be generally reduced in the ferrite after the applied field is removed, but the remaining (remanent) magnetization becomes a magnetic source that can generate an external magnetic field that can invade other structures such as a superconductor circuit in proximity to said magnetic structure.
Ferrite phase shifters using conducting microstrip meanderline techniques have been developed for several years. A standard ferrite-dielectric phase shifter includes a coupled microstrip meanderline fed by straight 50.OMEGA. feed lines. The meanderline, comprised of a standard conducting material such as copper, is deposited on a ferrite substrate which is magnetized in the direction of the meanderline elements. The gyromagnetic coupling between the magnetization of the ferrite and the magnetic field of the electromagnetic wave surrounding the meanderline conducting the microwave signal causes a phase shift of an amount proportional to the magnetization of the ferrite in the microwave signal traversing the meanderline.
The unit of efficiency for a phase shifter is known as the Figure of Merit ("FOM") which represents the differential phase shift in degrees induced in the electromagnetic wave conducted by the meanderline divided by the device insertion loss in decibels ("dB"). The differential phase shift is the change in phase that occurs when the direction of the magnetization is reversed. Several factors contribute to insertion loss, including: conductor resistance, gyromagnetic relaxation, and polaronic conductivity in the ferrite. Copper-based meanderline phase shifters in a frequency band from 5 to 6 GHz have been developed with a FOM on the order of 300 deg/dB as reported in:
Hansson, et al., "Planar Meanderline Ferrite-Dielectric Phase Shifter", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-29, No. 3, 208-215, (March, 1981). PA1 Denlinger, E. et al., "Superconducting Nonreciprocal Devices for Microwave Systems", IEEE Microwave and Guided Wave Letters, Vol. 2, No. II, 449-451 (November, 1992).
For the copper-based meanderline phase shifter design tested in the aforementioned Hansson article, the insertion loss was on the order of 2.0 dB, rendering the device impractical for many applications.
Scientists have experimented with replacing copper-based conductors with superconductors for application in ferrite microwave devices. One such study is reported in:
The study compared two Y-junction nonresonant microwave ferrite circulator designs, one employing a copper conductor and the other employing a superconductor. The stripline circulator design comprised a circular center conductor disposed between two ferrite disks magnetized by the magnetic field of an external magnet. The insertion loss of the copper device was 0.46 dB and the peak isolation was 25.3 dB at 77 Kelvin. For the superconductor sample, YBCO film was deposited on a dielectric substrate. YBCO is a high temperature superconductor with a critical temperature, T.sub.c, greater than 77 Kelvin. The insertion loss of the YBCO sample was 0.49 dB and the peak isolation was 34.1 dB at 77 Kelvin. Note that the insertion loss for the YBCO superconductor-based sample was slightly higher than the insertion loss for the copper-based sample. This was at least partly due to the magnetic field of the external magnet that invaded the superconductor and degraded the superconducting properties of the superconductor. Thus, the superconductor-based sample offered no significant improvement over the copper-based sample, and in fact had a higher insertion loss.